Stabilized borane-tetrahydrofuran solutions

ABSTRACT

The presence of an aliphatic, alicyclic, or cyclic sulfide stabilizes solutions of diborane in tetrahydrofuran, permitting the storage of such solutions for long periods of time at ambient temperatures. Such stabilized solutions can be prepared without handling diborane gas by treating a suspension of an ionic borohydride in tetrahydrofuran, containing an aliphatic, alicyclic, or cyclic sulfide, with boron trifluoride, followed by removal of the precipitated fluoroborate. Such solutions are excellent for hydroboration, and possess major advantages over borane-tetrahydrofuran solutions available previously in being highly stable.

United States Patent 1191 Brown [451 May 6,1975

[75] Inventor: Herbert C. Brown, West Lafayette,

Ind.

[73] Assignee: Aldrich-Boranes, Inc., Milwaukee,

, Wis.

[22] Filed: Feb. 22, 1973 211 Appl. No.: 334,642

[56] References Cited UNITED STATES PATENTS 2/1963 Brown 260/6065 B2/1963 Brown 260/6065 B 3,634,277 1/1972 Brown 252/188 PrimaryExaminerBenjamin R. Padgett Assistant Examiner-Irwin Gluck Attorney,Agent, or Firm-Merriam, Marshall, Shapiro & Klose [57] ABSTRACT Thepresence of an aliphatic, alicyclic, or cyclic sulfide stabilizessolutions of diborane in tetrahydrofuran, permitting the storage of suchsolutions for long periods of time at ambient temperatures. Suchstabilized solutions can be prepared without handling diborane gas bytreating a suspension of an ionic borohydride in tetrahydrofuran,containing an aliphatic, alicyclic, or cyclic sulfide, with borontrifluoride, followed by removal of the precipitated fluoroborate. Suchsolutions are excellent for hydroboration, and possess major advantagesover borane-tetrahydrofuran solutions avail able previously in beinghighly stable.

I 15 Claims, No Drawings STABILIZED BORANE-TETRAHYDROFURAN SOLUTIONSBACKGROUND OF THE INVENTION This invention relates to novel stabilizedsolutions of diborane in tetrahydrofuran containing as the stabilizingagent an aliphatic, alicyclic, or cyclic sulfide, and to the process ofpreparing the same. It also relates to a process for preparing suchstabilized solutions without handling gaseous diborane, by evolvingdiborane in situ in the presence of tetrahydrofuran containing analiphatic, alicyclic, or cyclic sulfide.

DESCRIPTION OF THE PRIOR ART Diborane, B H is a chemical with remarkableproperties. 1t reacts instantly with olefins in the presence of ethers,such as tetrahydrofuran, to fonn organoboranes.

This process is known as hydroboration," and is fully described in mypatent (H. C. Brown, US. Pat, No. 3,078,311, Feb. 19, 1963) and in mybook (H. C. Brown, Hydroboration, W. A. Benjamin Co., New York, 1962).

Diborane is also an exceedingly powerful, but selective hydrogenatingagent for functional groups. Whereas sodium borohydride is a basic-typereducing agent, diborane is an acidic-type reducing agent. Theavailability of both an acidic and a basic-type reducing agent makespossible numerous selective reductions or hydrogenations of functionalgroups.

Sodium borohydride, a crystalline solid, is a stable reagent which iseasily manufactured, stored, shipped, and used. Diborane, however, is agas, boiling at 92.5C, which is highly reactive towards air and moistureand consequently difficult to handle. Although attempts have been madeto compress diborane in tanks for shipment, this practice involves majordifficulty. Diborane in contact with the metal of the cylinders or tanksdecomposes spontaneously into hydrogen and higher hydrides of diborane.For this reason, it is rec ommended that such cylinders be refrigeratedand shipped or stored at low temperatures.

A possible solution to these difficulties would be to dissolve diboranein a suitable solvent and to ship the material in that form.Unfortunately, diborane does not possess adequate solubility inhydrocarbon solvents. It cannot be stored in such solvents as esters,dimethyl sulfoxide, or dimethylformamide, since these undergo relativelyrapid reduction. With amines, diborane reacts to form relatively stableamine-boranes, which fail to exhibit the desirable properties ofdiborane itself.

In ethers, such as diethyl ether, di-n-butyl ether, monoglyme (dimethylether of ethylene glycol), and diglyme (dimethyl ether of diethyleneglycol), the solubility of diborane is far too low to permit preparationof solutions containing enough diborane to be useful.

One suitable solvent for diborane is known. Diborane is highly solublein tetrahydrofuran, a cyclic ether. (.1. R. Elliott, W. L. Roth, G. F.Roedel and E. M. Boldebuck, J. Am. Chem. 500., 74, 5211 (1952)). In thissolvent it is possible to prepare solutions which are as concentrated as4 molar in borane (8H without exceeding one atmosphere of pressure.Moreover, such solutions are relatively safe to use. They can be exposedto the atmosphere without observable change, and can be poured throughthe air or into evaporating dishes and allowed to evaporate, withoutcatching fire.

Solutions of diborane in tetrahydrofuran are ideal for hydroborationsand selective hydrogenations. Unfortunately, two major problems arise inthe manufacture and shipment of such solutions.

The first problem stems from the dangerous nature of diborane. In thepast it has been customary to generate diborane by treating sodiumborohydride in diglyme solution with boron trifluoride-etherate.

2 5h The evolved diborane gas is then passed into the tetra hydrofuransolvent to make the solution. This process is undesirable in that itrequires the preparation and handling of large amounts of gaseousdiborane, an exceedingly reactive and hazardous material.

The second difficulty arises from the fact that solutions of diborane intetrahydrofuran have highly limited shelf-lives at ordinarytemperatures. Such solutions undergo reductive cleavage of thetetrahydrofuran by the diborane (.l. Kollonitsch, J. Am. Chem. 500., 83,1515 1961 As a result, the solutions lose 1 to 3 percent of theavailable diborane per day at ordinary temperatures (25 to 30C.), makingtheir manufacture, storage, and shipment impractical.

In my US. Pat. No. 3,634,277 ldescribed a means of stabilizing suchsolutions by adding thereto small amounts of sodium borohydride or otherionic borohydride in quantities not exceeding the solubility of theborohydride in the borane-tetrahydrofuran solution. Such solutions arethereby stabilized for periods of 4 to 8 weeks. Over longer periods,however, the solutions deteriorate. Another disadvantage is that thevapor pressure of diborane above such solutions is relatively high, sothat the solutions are usually manufactured in relatively lowconcentrations not exceeding 1 molar in 3H,. This means that largeamounts of solvent must be handled and shipped for a relatively smallquantity of diborane. In addition, in some reactions the included sodiumborohydride is active and interferes with the desired reaction of thediborane component. The present invention overcomes these disadvantages.

SUMMARY OF THE INVENTION 1 have discovered that the addition of asulfide to a solution of diborane in tetrahydrofuran greatly stabilizesthe solution, far more effectively and for longer periods of time thandoes sodium borohydride. Solutions stabilized in this manner areeffective reagents for hydroborations. In a preferred embodiment of theinvention the diborane is formed in situ in the presence oftetrahydrofuran containing a sulfide, by the reaction of BF with a metalor other ionic borohydride. Stabilized borane-tetrahydrofuran solutionscan be prepared in this manner without the necessity for handlinghazardous gaseous diborane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The sulfides which can be usedin the invention include aliphatic sulfides, such as dimethyl sulfide,methylethyl sulfide, diethyl sulfide, methylpropyl sulfide, methylbutylsulfide, and other lower alkyl sulfides; alicyclic sulfides, such asmethylcyclopentyl sulfide and methylcyclohexyl sulfide; cyclic sulfides,such as tetramethylene sulfide. pentamethylene sulfide andheptamethylene sulfide; and disulfides, such as l,3-dithiomethylpropane,CH SCH CH CH SCH The sulfides can contain inert substituents. such asmethoxy and methyl groups, as in CH OCH CH SCH and 2-methyltetramethylene sulfide. For greatest effectiveness, it isdesirable that the molecular weight of the sulfide be low, preferablybelow 200, so that relatively high molar concentrations can be achievedin the tetrahydrofuran solutions without markedly decreasing thetetrahydrofuran content of these solutions. Dimethyl sulfide ispreferred because of its low molecular weight and its low cost.

It is preferred that the concentration of the sulfide used in accordancewith the invention to stabilize diborane-tetrahydrofuran solutions beequal. on a molar basis, to the concentration of borane (EH in thesolution). Thus, for example, a solution that is 2 molar in borane isadvisedly also 2 molar in the sulfide. As the concentration of sulfideadded to the solution drops below that of the borane, the solutionbecomes increasingly less inhibited against borane loss. For purposes ofthe invention, any concentration of sulfide above about St) percent ofthe molar concentration of borane will yield a practical benefit instability, with the maximum effect being achieved at an equimolarconcentration. Solutions more concentrated in sulfide than equimolarwith borane can also be used, but are less desirable, since nosignificant increase in stability is achieved, while the tetrahydrofuranconcentration of the solution may be undesirably reduced.

The vapor pressure of diborane above such solutions is greatly reducedand tetrahydrofuran solutions that are as concentrated as 5 molar ormore in borane and sulfide are quite stable, easily handled, and readilyutilized for hydroboration.

The effectiveness of the sulfides of the invention in stabilizingdiborane-tetrahydrofuran solutions is exemplified by the data of TableI, showing the use of dimethyl sulfide for this purpose. Similar resultscan be achieved with the other sulfides contemplated for use in theinvention.

TABLE I STABILITY OF SOLUTIONS OF BORANE [N TETRAHYDROFURAN IN PRESENCEAND ABSENCE OF DlMETHYL SULFIDE AT C.

Concentration of BH, Concentration of BH:,

The stabilized sulfide-containing solutions of the invention can beprepared by dissolving gaseous diborane in a solution of the sulfide intetrahydrofuran, or by adding the sulfide to a solution of diborane intetrahydrofuran. However, these procedures require the handling ofdiborane gas, a highly reactive hazardous material. l have discoveredthat highly stabilized diborane solutions can be prepared by addingboron trifluoride as a gas or a complex with an organic base to asuspension of an ionic borohydride in tetrahydrofuran containing thedesired sulfide for stabilization, liberating diborane which passes intosolution. Although a wide variety of ionic borohydrides, such as lithiumborohydride and tetramethylammonium borohydride, can be used, I prefersodium borohydride because of its economy. In this way the stabilizedsolutions of the inven tion can be prepared without the necessity ofhandling gaseous diborane.

Preparation of the stabilized diborane solutions of the invention isillustrated by the following examples.

EXAMPLE 1 In a 2-liter flask flushed with nitrogen was placed 0.80 literof dry tetrahydrofuran. Sodium borohydride, 57 g. 1.5 moles), was addedand the mixture was vig orously stirred to keep the salt in solution. 2moles of ethyl ether-boron trifluoride was added slowly to thewell-stirred solution cooled in an ice salt bath to maintain thetemperature at 0. After all ofthe etherate have been added, the solutionwas stirred for an additional hour, a filter stick was then introducedand the solution pushed through the filter stick by nitrogen pressureinto the storage bottle. To the cold filtered solution was added I24 g.(20 moles) of dimethyl sulfide and a small amount of tetrahydrofuran tomake the total volume 1.0 liter. The yield was almost quantitative, thesolution 1.0 liter) analyzing about 1.96 molar in borane. No change inthe borane content of the solution, maintained at 25C. under nitrogen,occurred over a storage period of eight weeks.

As a control, a solution of diborane was prepared following the aboveprocedure, except that no dimethyl sulfide was added. The controlsolution was not stable in storage, but rather underwent a loss ofapproximately 2 percent of active hydrogen per day.

EXAMPLE 2 Dimethyl sulfide, 124 g. (2.0 moles), was dissolved in 700 mlof tetrahydrofuran. One mole of diborane was generated from borontrifluoride etherate and sodium borohydride in diglyme and the gas waspassed into the tetrahydrofuran solution. Sufficient tetrahydrofuran wasadded to make the volume 1 liter. Analysis revealed the boraneconcentration to be 2.0 molar. No change in borane content was observedover 8 weeks.

EXAMPLE 3 Tetramethylene sulfide (tetrahydrothiophene l76 g. (2.0moles), was dissolved in 500 ml of tetrahydrofuran. One mole of diboranewas generated from boron trifluoride etherate and sodium borohydride indiglyme and the gas was passed into the tetrahydrofuran solution.Sufficient tetrahydrofuran was added to make the volume one liter.Analysis revealed the borane concentration to be 2.0 molar. No change inborane content was observed over two weeks.

EXAMPLE 4 A 2-liter flask was immersed in an ice bath. In the flask wasplaced 58 g. (1.5 moles) of sodium borohydride (98 percent), 625 cc oftetrahydrofuran, and 124 g. of dimethyl sulfide. The flask was flushedwith nitrogen and a static nitrogen pressure maintained. Through adropping funnel was added to the stirred mixture 225 ml (2.0 moles) oftetrahydrofuran-boron trifluoride over one hour. The mixture was broughtto room temperature and allowed to stand overnight. The sodiumfluoroborate settled, yielding a clear solution. The clear solution waspushed with nitrogen through a hypodermic needle into storage flasks.Analysis revealed the borane concentration to be 1.99 molar (theoretical2.00), indicating a yield of 99.5 percent of the available hydride.

EXAMPLE 5 The preparation can also be carried out with the reactants atThe reaction is slightly exothermic, so water cooling should be used.

A 2-liter flask was connected to a dropping funnel, a stirrer, and anitrogen source. In the flask was placed 625 cc of tetrahydrofuran and58 g. l.5 moles) of sodium borohydride (98 percent). The flask andcontents were flushed with nitrogen and a static nitrogen atmospheremaintained. Then 125 g. of dimethyl sulfide was added. A water bath wasplaced about the flask. To the stirred reaction mixture was added 225 ml(2.0 moles) of tetrahydrofuranboron trifluoride over a period of 1 hour,maintaining the temperature at approximately 25. The reaction wasallowed to settle overnight. The clear solution was transferred by meansof a hypodermic needle into a storage flask. Analysis revealed theborane concentration to be 1.99 molar (theoretical 2.00), indicating ayield of 99.5 percent of the available hydride.

In addition to the specific sulfides used in the above examples, otherpreviously described sulfides can be used with similar results.

The stabilized diborane solutions of the invention are highly effectivefor hydroborations and other reactions. For example, a solution ofborane in tetrahydrofruan was used to hydroborate a number of olefins at25. The reaction mixtures were allowed to stand for l hour, and thenoxidized with alkaline hydrogen peroxide. 1- Hexene gave a 99.5 percentyield of alcohols, 94.6 percent of l-hexanol, 5.4 percent of Z-hexanol.cis-3- Hexene gave an 88 percent yield of pure 3-hexan0l. Norbornene wasconverted into the Z-norbornanol in 98% yield, 99 percent exo and 1percent endo. Finally, l-methylcyclopentene gave a 95 percent yield ofits alcoho], 98.9 percent trans-Z-methylcyclopentanol and 1.1 percent ofl-methyLl-cyclopentanol.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

l. A method for stabilizing a solution of diborane in tetrahydrofuranwhich comprises incorporating in said solution an effectiveconcentration of an alkyl, alicyclic, or cyclic sulfide.

2. The method of claim 1 in which said sulfide has a molecular weightless than about 200, and is present in said solution in an amount of atleast about one mole per mole of borane.

3. The method of claim 2 in which said sulfide is dimethyl sulfide.

4. The method of claim 2 in which said sulfide is diethyl sulfide.

S. The method of claim 2 in which said sulfide is tet rarnethylenesulfide.

6. A solution of diborane is tetrahydrofuran which contains as astabilizing agent an effective concentration of an alkyl, alicyclic orcyclic sulfide.

7. The solution of claim 6 wherein said sulfide has a molecular weightless than about 200 and is present in said solution in an amount of atleast 1 mole per mole of borane.

8. The solution of claim 7 wherein said sulfide is dimethyl sulfide.

9. The solution of claim 7 wherein said sulfide is diethyl sulfide.

10. The solution of claim 7 wherein said sulfide is tetramethylenesulfide.

11. A process for preparing a stabilized solution of diborane intetrahydrofuran which comprises reacting an ionic borohydride with BF inthe presence of tetrahydrofuran containing an alkyl, alicyclic or cyclicsulfide in an amount effective to stabilize the resulting solution, andseparating the stabilized solution from the reaction mixture.

12. The method of claim 10 wherein said ionic borohydride is sodiumborohydride.

13. The method of claim 12 in which said sulfide is dimethyl sulfide andis present in said stabilized solution in an amount of at least one moleper mole of borane.

14. The method of claim 12 in which said sulfide is diethyl sulfide andis present in said stabilized solution in an amount of at least 1 moleper mole of borane.

15. The method of claim 12 in which said sulfide is tetramethylenesulfide and is present in said stabilized solution in an amount of atleast l mole per mole of bo rane.

1. A METHOD FOR STABLIZING A SOLUTION OF DIBORANE IN TETRAHYDROFURANWHICH COMPRISES INCORPORATING IN SAID SOLUTION AN EFFECTIVECONCENTRATION OF AN ALKYL, ALICYCLIC, OR CYCLIC SULFIDE.
 2. The methodof claim 1 in which said sulfide has a molecular weight less than about200, and is present in said solution in an amount of at least about onemole per mole of borane.
 3. The method of claim 2 in which said sulfideis dimethyl sulfide.
 4. The method of claim 2 in which said sulfide isdiethyl sulfide.
 5. The method of claim 2 in which said sulfide istetramethylene sulfide.
 6. A solution of diborane is tetrahydrofuranwhich contains as a stabilizing agent an effective concentration of analkyl, alicyclic or cyclic sulfide.
 7. The solution of claim 6 whereinsaid sulfide has a molecular weight less than about 200 and is presentin said solution in an amount of at least 1 mole per mole of borane. 8.The solution of claim 7 wherein said sulfide is dimethyl sulfide.
 9. Thesolution of claim 7 wherein said sulfide is diethyl sulfide.
 10. Thesolution of claim 7 wherein said sulfide is tetramethylene sulfide. 11.A process for preparing a stabilized solution of diborane intetrahydrofuran which comprises reacting an ionic borohydride with BF3in the presence of tetrahydrofuran containing an alkyl, alicyclic orcyclic sulfide in an amount effective to stabilize the resultingsolution, and separating the stabilized solution from the reactionmixture.
 12. The method of claim 10 wherein said ionic borohydride issodium borohydride.
 13. The method of claim 12 in which said sulfide isdimethyl sulfide and is present in said stabilized solution in an amountof at least one mole per mole of borane.
 14. The method of claim 12 inwhich said sulfide is diethyl sulfide and is present in said stabilizedsolution in an amount of at least 1 mole per mole of borane.
 15. Themethod of claim 12 in which said sulfide is tetramethylene sulfide andis present in said stabilized solution in an amount of at least 1 moleper mole of borane.